Consider a 0.1 mm diameter cylindrical metal filament that thermionically emits electrons over a length of 1 cm. Assume the reflection coefficient of the metal r = = 0 and that emission comes from the entire surface of the cylinder minus the circular ends. A measurement of the temperature and emission current of the filament shows that at 2000 K, the filament emits a total of 38.3 A. The filament is made of one of the elements in the table below. Based on the above information and the following table of work function data, of which metal is this filament made? Metal Ca Mg Ta Cr Pt Work function, 6(eV) 3.2 3.6 4.2 4.6 5.3